Lattice-matched, large-grain HTS films for reproducible Josephson junctions

Several novel techniques are now available to dramatically increase the grain size of epitaxial Rare-earth-BCO films so they are effectively single crystals. Our motivation is to use single-crystal films for base electrodes of edge SNS junctions to improve their reproducibility. We employed the technique of "tri-phase epitaxy" to grow films by liquid phase epitaxy in a pulsed laser deposition system and examined the surface morphology and crystal structure of the films. We tried several techniques for in-situ monitoring of a Ba-Cu liquid flux on the surface of the film during growth - including resistivity and emissivity measurements - but found that the only way to be certain that we had a liquid phase on the sample surface was from the dendritic crystal patterns that formed after cooling the samples. We observed series of small cracks in the resulting films, presumably due to stress from lattice-match and thermal expansion mismatch with the substrates so we developed a solid solution of YBa₂Cu₃O₇ and NdBa₂Cu₃O₇ to match the lattice of our substrates to better than 0.1%.